Adjustment and testing of crystal rectifiers



E. 0L KEIZER ADJUSTMENT AND-TESTING OF CRYSTAL REGTIFIERS Filed Nov. 29, 1943 Gt'torneg Patentedv Oct. 8, 1946 ADJUSTMENT AND TESTING OF CRYSTAL RECTIFIERS Eugene 0. Keizer, Princeton,

N. J., assigner to Radio Corporation of America, a corporation of Delaware Application November 29, 1,943, Serial No. A512,150

(el. 25o- 20) 9 Claims.

This invention relates to microwave apparatus and more particularly to an improved method of and means for adjusting and testing crystal rectifiers under conditions comparable to their use in microwave apparatus.

Various types of crystal rectiflers constitute efficient and extremely sensitive detectors of low magnitude microwave energy. The selection and adjustment of catwhisker contact of crystal detectors at super-high frequencies involves different technique than that found satisfactory for detectors of ordinary radio frequencies, since the relatively low signal levels at super-high frequencies necessitate a relatively high signal-to-noise ratio in the microwave detector for satisfactory reception.

One type of microwave crystal detector which provides satisfactory operation at super-high frequencies comprises a silicon crystal having a knife edge for contact with a tungsten catwhisker. A preferred embodiment of this type of crystal and the mounting thereof is described in the copending application of Wendell L. Carlson, Ser. No. 507,755, filed October 26, 1943, and assigned to the same assignee as the instant application.

Briefly, the instant invention comprises an improvedmethod of and means for employing a microwave crystal detector in a circuit which will provide comparable operation to that'encountered in a conventional microwave receiver, and wherein an indication is obtained of the signalto-noise efficiency ratio of the crystal detector.

One embodiment of the invention comprises means for applying low frequency amplitude modulated microwave signals and unmodulated microwave signals of a different frequency to a mixer circuit which includes the crystal to be ltested and adjusted. The modulated beat frequency signals derived from the detector crystal 'mixen which include extraneous or noise current components, are appli-ed to a conventional intermediate frequency amplier. The amplified modulated beat frequency signals are detected by, for example, a diode detector and applied to the input of a conventional low-pass filter network. The filtered signals derived from the lowpass network are amplified at audio frequencies, corresponding to the original modulation frequency, and applied to an automatic volume control circuit to provide a control voltage for regulating the average gain of the intermediate frequencycircuits so that the amplified audio output is maintained substantially constant. An indicator responsive to the control voltage applied to the intermediate frequency amplifier provides an indication of the signal-to-noise ratio of the signals derived from the crystal mixer, since the constant magnitude audio output signals control the average magnitude of the intermediate frequency currents which are modulatedby both noise and the audio signal components. t

A second embodiment of the invention com-k prises means for applying unmodulated and sawtooth frequency-modulated currents of microwave frequencies of different carrier frequencies to a mixer circuit including a crystal detector to be tested. The frequency modulated beat frequency signals derived from the mixer, which include extraneous or noise current components, are applied to a conventional intermediate frequency amplifier which has a band-pass of the order, for example, of one-third the frequency modulation band width. The signals derived from the intermediate frequency amplifier comprise pulses of intermediate frequency energy which are applied to a diode detector from which is selected the low frequency modulation component. A conventional automatic volume control circuit responsive to the output of the diode detector controls the average gain of the intermediate frequency amplifier.v The low frequency signals derived from the diode detector are applied through a conventional low-pass filter to the input of an audio amplifier. The amplified signals derived from the audio amplifier are applied to an, output signal indicator and to .the vertical deflection' electrodes of a cathode ray oscillograph.Y `The horizontal deflection electrodes of the oscilloscope are connected to the source of saw-tooth frequency modulation signals to .provide a timing voltage for the oscilloscope.,`

Among the objects of the invention are to provide an. improved method of and means for testing microwave detectors. Another object of the invention is to provide an improved method of and means for testing and adjusting crystal rectifiers under conditions comparable to their use in lmicrowave receivers. A further object of the invention is to provide an improved method of and means for testing microwave detectors by employing them as first detectors in a superheterodyne circuit and indicating the signal-to-noise efficiency ratio of the microwave first detector.

Other objects of the invention include im= proved methods of and means for testing microwave detectors by applying unmodulated and amplitude modulated microwave signals of Ydifferent frequency to said detector and indicating the signal-to-noise ratio of the circuit including said detector. An additional object of the invention is to provide an improved method of and means for employing unmodulated and frequency modulated signals of microwave frequencies to 'a mixer circuit including the detector to be tested, applying beat frequencies derived from said detector to a relatively narrow band intermediate frequency amplier, and indicating vthe signalto-noise ratio of the signals derived from said detector and also indicating the frequency modulation characteristics of the frequency modulated source.

The invention will be further described by reference to the accompanying drawing of which Figure 1 is a schematic block circuit diagram of one embodiment thereof, Figure. 2 is a schematic block circuit diagram of a second and preferred embodiment thereof and Figure 3 is a schematic circuit diagram of a mixer circuit including a crystal detector. Similar reference characters areA applied to similar elements throughout the drawing.

Referring to Figure 1 of the drawing, a. signal generator l, such as, for example, aconventional magnetron oscillator, is amplitude modulated by a modulation source 3, such as, for example, a source of square wave modulation pulses. It should be understood that the modulation signals may be any other desired wave shape. The amplitude modulated signals from the signal generator l and unmodulatedmicrowave.signals having a different carrier. frequency derived from a second. local oscillator 5, such as, for example, a second magnetronoscillator, are, applied to a detector crystal. mixer circuit 'l which includes a microwave crystal detector which is to be tested.

Square wave amplitude modulated beat. frequency signals derived from the crystal mixer circuit 1 are appliedtothe input of a conventional tuned intermediate frequency amplifier 9 from which are derived signals havingY a wave form indicated by the graph (l l). In the absence of signais fromeither of the lo,cal.signal generators l or 5, noise signals .indicated by the highfrequency oscillations I3l will be derivedfrom the intermediate frequency amplifier 9.. When the modulated beat frequency signals are derived fromthe crystal mixer circuit l, these noise frequency oscillations willbe modulated by the square wave modulation pulses as indicated by the portion ofthe graph l5.

The modulated beatv frequency signals, including the noise. components, areV applied to the input of a diode detector l1 from which modulation signals characterizedV by the graph I9. are derived. These signals will include some noise components as indicated by. the graph portion 2l and modulation pulses as indicated by the graph portion 23. The signals derived from the diode detector l1 are applied to the input of a conventional low-pass filter 25 from which is derived a substantially noise-free modulation signal characterized by the graph 21. rEhe signals 21, derived from the output of the low-pass. filter 25, are, amplified by means of a conventional audio amplifier 29 to derive alternating potentials, characterized. by the graph 3|-, which are applied to control a conventional automatic volume control circuit 33. Control voltages derived from the output of the automatic control circuit 33 are applied, through an indicator 35, to control the average gain of the intermediate frequency amplifier 9 thereby to maintain the audio output essentially constant.

Since the average gain of the intermediate frequency amplifier 9 is varied so the audio output is maintained at a uniform magnitude, the control voltage applied to the intermediate frequency amplier, and indicated on the indicator 35, will be a measure of the relative magnitudes of the noise signal components I3 and the regulated maximum modulation signal components l5, and hence will indicate the signal-to-noise ratio of the crystal mixer. By substituting different crystal detectors in the crystal mixer circuit, the relative signal-to-noise eliiciency of the various detectors may be determined.

One disadvantage of the circuit thus described is that unless the carrier frequencies of the signal generators i and 5 are maintained at constant values, the magnitudes of the modulated beat frequency signals derived from the intermediatebeat-frequency amplifier 9 will vary as a function of the band-pass characteristics of the amplifier 9, and hence will provide inaccurate indications of the signal-to-noise ratiov of thev crystal, employed in the crystal mixer 1. I

Referring to Figure 2 of the drawing, an` uninodulated microwave signal generator of the type described heretofore, is connected to then input 0f a crystal type microwave mixer l. Asecond local microwave oscillator 5, of any type known, in the art, is frequency modulated by means of. a, sawtooth frequency modulator 3 to apply frequency modulated signals toasecond input circuit of the crystal microwave mixer l. Frequency modulated beat frequency signals, derived from thevcrystal mixer 1, are applied tothe input of a conventionalintermediate amplifier 9 having aba-ndpass, for example, of the order of one-third the frequency bandof the frequency modulated beat frequency signals. The signals derived from the intermediate frequency amplifier 9 comprise pulses of intermediate frequency energy corresponding to the portion, of the frequency modulated intermediate frequency spectrum transmitted bythe intermediate frequency band-pass circuits. Y

These pulses of intermediate frequency energy 'are applied to a diode` detector IT, from which is derived amplitude modulated pulses including noise components, and in which the frequency 0f the pulse,modulationcorresponds tothe frequen.- cy of the modulation source 3. A portionv ofthe output signals derived from the diode detector l1 is appliedto control a conventional automaticvolume control circuit.33 to provide acontrolsignal which is applied tocontrol the average gain. of the intermediate-frequency amplifier.

Hence, theY total modulation signal components and noise signalcomponents derived. from thein.- termediate frequency amplifier a and the diode detector l1 are averagedby the automatic volume control circuit 33. The modulation and noise signais derived from the diode detector I1 also are applied, through a conventional low-pass filter 25, to the input of a conventional audio amplifier 2.9.from which is derived. alternating modulation signals` having amplitudes characteristic of the relative magnitude of the modulation signal currents and the average energy transmitted by the intermediate frequency amplifier and diode detector. The signals derived from the audio amplifier 29 are applied to an output meter 35^which indicates the magnitude of the signals derived from the audio amplifier 29 and hencethesgnalto-noise ratio of signals applied to the input` of the intermediate frequency amplifier 9- from the crystal mixer 1. The alternating signalsv derived from the audio amplifier 25 also are applied to the vertical deiiection electrodes of a conventional cathode ray oscilloscope 31 to provide vertical deflection of the cathode ray in accordance with the instantaneous values of the currents derived from the audio amplifier 29. The saw-tooth fijequency modulator 3 is connected to the horizontal deflection electrodes of the cathode ray oscilloscope 31 to provide a timing voltage to indicate the relative portion of the frequency modulated spectrum of the intermediate frequency signals which is transmitted by the band-pass circuits of the intermediate frequency amplier 9. It should be understood that,'if desired, the automatic volume control circuit may be made responsive to the amplified audio output signals in the same manner as described in the device of Figure 1.

In this second embodiment of the invention, it is immaterial whether slight frequency variations occur in the microwave signal generators l and 5, since the only effect thereof upon the output indications will be to vary the horizontal position of the deflection pattern along the timing axis of the cathode ray oscilloscope. Hence, much more accurate indications of the signal-tonose ratio of the crystal miXer 1 may be obtained since the intermediate frequency amplifier does not affect the signal-to-noise indication as a function of minor variations in the microwave carrier frequencies.

Figure 3 shows a typical mixer circuit comprising a tuned or untuned transformer 40 having a first primary winding 4I connected to the signal generator I and a second primary winding 42 connected to the local oscillator 5. A secondary winding 43 is serially connected through a crystal detector 44, or a thermionic tube detector to output terminals 45 which provide beat frequency signal currents. A capacitor 46 l'by-passes radio frequency currents around the output terminals 45.

Thus the invention described comprises improved methods of and means for testing the signal-to-noise ratio of microwave detectors under conditions comparable to their use in conventional microwave receivers, and provide means whereby the individual microwave detectors may be adjusted under comparable operating conditions for maximum efficiency.

I claim as my invention:

l. The method of testing a microwave detector providing signal and noise output current components comprising employing said detector as a miXer in a microwave superheterodyne circuit,

, applying unmodulated and modulated microwave currents of dierent frequencies to said detector, deriving therefrom modulated difference-frequency currents and indicating the efficiency of said detector in terms of the modulation signalto-circuit noise current ratio of said derived currents.

2. The method of testing a microwave detector providing both signal and noise output current components comprising generating unmodulated microwave currents, generating modulated microwave currents, applying said modulated and said unmodulated currents to said detector, deriving from said detector noise currents and modulated currents of the difference-frequency of said microwave currents, rectifying and selecting the modulation current component from said noise and said modulated difference-frequency currents, deriving an automatic control voltage from said selected modulation current, applying said derived control voltage to regulate the average magnitudes of said noise and v'said difference-frequency currents, and indicating the eiiiciency of said detector in response to the magnitudes of said control voltage. y y

3. The method of testing a microwave'dete'ctor providing both signal and noise output current components comprising generating unmodulated microwave currents, generating amplitudef`modulated microwave currents, applying said modulated and said unmodulated currents to said vdetector, deriving from said detectornoise currents and modulated currents of the difference-frequency of said microwave currents, rectifying and selecting the modulation current component from said noise and said modulateddifference-frequency currents, deriving an automatic control voltage from said selected modulation current', applying said derived control voltage to regulate the average magnitudes of said difference-frequency currents, and indicating the signal-tonoise efficiency ratio of said detector in response to the magnitudes of said control voltage.

4. The method of -testing a microwave detector providing both signal and noise output current components comprising generating unmodulated microwave currents, generating frequency modulated microwave currents, applying said frequency modulated and said unmodulated currents to said detector, deriving from said detector noise currents and amplitude modulated currents of predetermined difference-frequencies of said microwave currents, rectifying and selecting the amplitude modulation current component from said noise and said modulated difference-frequency currents, deriving an automatic control voltage from said derived currents, applying said control voltage to regulate the average magnitudes of said noise and said difference-frequency currents, and indicating the signal-to-noise eidciency ratio of said detector in response to the magnitudes of said selected modulation currents.

5. The method described in claim 4 including separately indicating the frequency relation of said derived predetermined difference-frequencies with respect to the complete frequency modulation difference-frequency spectrum.

6. Apparatus for testing microwave detectors providing both signal and noise output current components including means for generating unmodulated microwave currents, means for generating modulated microwave currents, a microwave detector, means for applying said modulated and said unmodulated currents to said detector, means for deriving from said detector noise currents and modulated currents of the differencefrequency of said microwave currents, means for rectifying and selecting the modulation current component from said noise and said modulated difference-frequency currents, means for automatically regulating said detected currents, and means responsive to said selected currents for indicating the signal-to-noise eiiiciency ratio of said detector.

7. Apparatus for testing microwave detectors providing both signal and noise output current components including means for generating unmodulated microwave currents, means for generating modulated microwave currents, a microwave detector, means for applying said modulated and said unmodulated currents to said detector for deriving from said detector noise currents and modulated currents of the difference-frequency of said microwave currents, means for rectifying and selecting the modulation current component from said rectified noise and said modulated difEerence-frequency currents, means for automatically regulating the magnitudeofatleast one ofsaid detected current components,4 and means responsive to said selected currents. for indicating the signal-to-noise-.eiciency ratioo'f said detector. Y i

8. Apparatus for testing-microwave detectors providing both'signal and nnoise output. current components including means for generatingun-` modulated microwave currentsmeans for generating frequency modulated microwave currents, av microwave detector, means for applying said modulated and said unmodulated currents to said detector for deriving from said detector noise currents and frequency modulated currents of the difference-frequency of said microwave currents, lter means for deriving a predetermined portion of said frequency-modulated difference-frequency 

